The direct answer to your question is that, whichever procedure you are using to make your MPO necessarily includes "h" as a parameter. (Say using AutoMPO, where "h" appears as a coefficient.) So then to change h, you just re-create the MPO again from the beginning, using a different value for h.
However, the other answer I would give to your question is to say, I don't recommend doing a lot of different calculations (I assume DMRG?) in a loop. Instead, I think it's a much better practice to do each calculation separately, and carefully study each one. How well did it go? Were all indications that it converged? Is the system approaching a phase boundary where perhaps the initial state and number of sweeps needed for convergence might need to be changed?